Base station for controlling initial power ramp-up using short codes

ABSTRACT

A base station for controlling transmission power during the establishment of a communication channel utilizes the reception of a short code during initial power ramp-up. The short code is a sequence for detection by the base station which has a much shorter period than a conventional access code. The ramp-up starts from a power level that is lower than the required power level for detection by the base station. The power of the short code is quickly increased until the signal is detected by the base station. Once the base station detects the short code, it transmits an indication that the short code has been detected.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application is a continuation of application Ser. No.09/721,034, filed Nov. 22, 2000, which application is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates generally to CDMA communicationsystems. More specifically, the present invention relates to a CDMAcommunication system which utilizes the transmission of short codes fromsubscriber units to a base station to reduce the time required for thebase station to detect the signal from a subscriber unit. The improveddetection time allows a faster ramp-up of the initial transmit powerfrom the subscriber units while reducing the unnecessary powerovershoot.

[0004] 2. Description of Related Art

[0005] The use of wireless telecommunication systems has growndramatically in the last decade as the reliability and capacity of thesystems have improved. Wireless communication systems are being utilizedin a variety of applications where land line based systems areimpractical or impossible to use. Applications of wirelesscommunications include cellular phone communications, communications inremote locations, and temporary communications for disaster recovery.Wireless communication systems have also become an economically viablealternative to replacing aging telephone lines and outdated telephoneequipment.

[0006] The portion of the RF spectrum available for use by wirelesscommunication systems is a critical resource. The RF spectrum must beshared among all commercial, governmental and military applications.There is a constant desire to improve the efficiency of wirelesscommunication systems in order to increase system capacity.

[0007] Code division multiple access (CDMA) wireless communicationsystems have shown particular promise in this area. Although moretraditional time division multiple access (TDMA) and frequency divisionmultiple access (FDMA) systems have improved using the latesttechnological advances, CDMA systems, in particular Broadband CodeDivision Multiple Access™ (B-CDMA™) systems, have significant advantagesover TDMA and FDMA systems. This efficiency is due to the improvedcoding and modulation density, interference rejection and multipathtolerance of B-CDMA™ systems, as well as reuse of the same spectrum inevery communication cell. The format of CDMA communication signals alsomakes it extremely difficult to intercept calls, thereby ensuringgreater privacy for callers and providing greater immunity againstfraud.

[0008] In a CDMA system, the same portion of the frequency spectrum isused for communication by all subscriber units. Each subscriber unit'sbaseband data signal is multiplied by a code sequence, called the“spreading code”, which has a much higher rate than the data. The ratioof the spreading code rate to the data symbol rate is called the“spreading factor” or the “processing gain”. This coding results in amuch wider transmission spectrum than the spectrum of the baseband datasignal, hence the technique is called “spread spectrum”. Subscriberunits and their communications can be discriminated by assigning aunique spreading code to each communication link which is called a CDMAchannel. Since all communications are sent over the same frequency band,each CDMA communication overlaps communications from other subscriberunits and noise-related signals in both frequency and time.

[0009] The use of the same frequency spectrum by a plurality ofsubscriber units increases the efficiency of the system. However, italso causes a gradual degradation of the performance of the system asthe number of users increase. Each subscriber unit detects communicationsignals with its unique spreading code as valid signals and all othersignals are viewed as noise. The stronger the signal from a subscriberunit arrives at the base station, the more interference the base stationexperiences when receiving and demodulating signals from othersubscriber units. Ultimately, the power from one subscriber unit may begreat enough to terminate communications of other subscriber units.Accordingly, it is extremely important in wireless CDMA communicationsystems to control the transmission power of all subscriber units. Thisis best accomplished by using a closed loop power control algorithm oncea communication link is established. A detailed explanation of such aclosed loop algorithm is disclosed in U.S. Patent Application entitledCode Division Multiple Access (CDMA) System and Method filedconcurrently herewith, which is incorporated by reference as if fullyset forth.

[0010] The control of transmission power is particularly critical when asubscriber unit is attempting to initiate communications with a basestation and a power control loop has not yet been established.Typically, the transmission power required from a subscriber unitchanges continuously as a function of the propagation loss, interferencefrom other subscribers, channel noise, fading and other channelcharacteristics. Therefore, a subscriber unit does not know the powerlevel at which it should start transmitting. If the subscriber unitbegins transmitting at a power level that is too high, it may interferewith the communications of other subscriber units and may even terminatethe communications of other subscriber units. If the initialtransmission power level is too low, the subscriber unit will not bedetected by the base station and a communication link will not beestablished.

[0011] There are many methods for controlling transmission power in aCDMA communication system. For example, U.S. Pat. No. 5,056,109(Gilhousen et al.) discloses a transmission power control system whereinthe transmission power of the subscriber unit is based upon periodicsignal measurements from both the subscriber unit and the base station.The base station transmits a pilot signal to all subscriber units whichanalyze the received pilot signal, estimate the power loss in thetransmitted signal and adjust their transmission power accordingly. Eachsubscriber unit includes a non-linear loss output filter which preventssudden increases in power which would cause interference to othersubscriber units. This method is too complex to permit a base station toquickly acquire a subscriber unit while limiting the interference toother subscriber units. In addition, the propagation losses,interference and noise levels experienced in a forward link(transmission from the base station to a subscriber unit) is often notthe same as in a reverse link (transmission from a subscriber unit tothe base station). Reverse link power estimates based on forward linklosses are not precise.

[0012] Many other types of prior art transmission power control systemsrequire complex control signaling between communicating units orpreselected transmission values to control transmission power. Thesepower control techniques are inflexible and often impractical toimplement.

[0013] Accordingly, there is a need for an efficient method ofcontrolling the initial ramp-up of transmission power by subscriberunits in a wireless CDMA communication system.

SUMMARY OF THE INVENTION

[0014] The present invention comprises a novel method of controllingtransmission power during the establishment of a channel in a CDMAcommunication system by utilizing the transmission of a short code froma subscriber unit to a base station during initial power ramp-up. Theshort code is a sequence for detection by the base station which has amuch shorter period than a conventional spreading code. The ramp-upstarts from a power level that is guaranteed to be lower than therequired power level for detection by the base station. The subscriberunit quickly increases transmission power while repeatedly transmittingthe short code until the signal is detected by the base station. Oncethe base station detects the short code, it sends an indication to thesubscriber unit to cease increasing transmission power. The use of shortcodes limits power overshoot and interference to other subscriberstations and permits the base station to quickly synchronize to thespreading code used by the subscriber unit.

[0015] Accordingly, it is an object of the present invention to providean improved technique for controlling power ramp-up during establishmentof a communication channel between a CDMA subscriber unit and basestation.

[0016] Other objects and advantages of the present invention will becomeapparent after reading the description of a presently preferredembodiment.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic overview of a code division multiple accesscommunication system in accordance with the present invention;

[0018]FIG. 2 is a diagram showing the operating range of a base station;

[0019]FIG. 3 is a timing diagram of communication signals between a basestation and a subscriber unit;

[0020]FIG. 4 is a flow diagram of the establishment of a communicationchannel between a base station and a subscriber unit;

[0021]FIG. 5 is a graph of the transmission power output from asubscriber unit;

[0022]FIGS. 6A and 6B are flow diagrams of the establishment of acommunication channel between a base station and a subscriber unit inaccordance with the preferred embodiment of the present invention usingshort codes;

[0023]FIG. 7 is a graph of the transmission power output from asubscriber unit using short codes;

[0024]FIG. 8 shows the adaptive selection of short codes;

[0025]FIG. 9 is a block diagram of a base station in accordance with thepresent invention;

[0026]FIG. 10 is a block diagram of the subscriber unit in accordancewith the present invention; and

[0027]FIGS. 11A and 11B are flow diagrams of the ramp-up procedureimplemented in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0028] The preferred embodiment will be described with reference to thedrawing figures where identical numerals represent similar elementsthroughout.

[0029] A communication network 10 embodying the present invention isshown in FIG. 1. The communication network 10 generally comprises one ormore base stations 14, each of which is in wireless communication with aplurality of subscriber units 16, which may be fixed or mobile. Eachsubscriber unit 16 communicates with either the closest base station 14or the base station 14 which provides the strongest communicationsignal. The base stations 14 also communicate with a base stationcontroller 20, which coordinates communications among base stations 14.The communication network 10 may also be connected to a public switchedtelephone network (PSTN) 22, wherein the base station controller 20 alsocoordinates communications between the base stations 14 and the PSTN 22.Preferably, each base station 14 communicates with the base stationcontroller 20 over a wireless link, although a land line may also beprovided. A land line is particularly applicable when a base station 14is in close proximity to the base station controller 20.

[0030] The base station controller 20 performs several functions.Primarily, the base station controller 20 provides all of theoperations, administrative and maintenance (OA&M) signaling associatedwith establishing and maintaining all of the wireless communicationsbetween the subscriber units 16, the base stations 14, and the basestation controller 20. The base station controller 20 also provides aninterface between the wireless communication system 10 and the PSTN 22.This interface includes multiplexing and demultiplexing of thecommunication signals that enter and leave the system 10 via the basestation controller 20. Although the wireless communication system 10 isshown employing antennas to transmit RF signals, one skilled in the artshould recognize that communications may be accomplished via microwaveor satellite uplinks. Additionally, the functions of the base stationcontroller 20 may be combined with a base station 14 to form a “masterbase station”.

[0031] Referring to FIG. 2, the propagation of signals between a basestation 14 and a plurality of subscriber units 16 is shown. A two-waycommunication channel (link) 18 comprises a signal transmitted 20 (Tx)from the base station 14 to the subscriber unit 16 and a signal received22 (Rx) by the base station 14 from the subscriber unit 16. The Txsignal 20 is transmitted from the base station 14 and is received by thesubscriber unit 16 after a propagation delay At. Similarly, the Rxsignal originates at the subscriber unit 16 and terminates at the basestation 14 after a further propagation delay At. Accordingly, the roundtrip propagation delay is 2Δt. In the preferred embodiment, the basestation 14 has an operating range of approximately 30 kilometers. Theround trip propagation delay 24 associated with a subscriber unit 16 atthe maximum operating range is 200 microseconds.

[0032] It should be apparent to those of skill in the art that theestablishment of a communication channel between a base station and asubscriber unit is a complex procedure involving many tasks performed bythe base station and the subscriber unit which are outside the scope ofthe present invention. The present invention is directed to initialpower ramp-up and synchronization during the establishment of acommunication channel.

[0033] Referring to FIG. 3, the signaling between a base station 14 anda subscriber unit 16 is shown. In accordance with the present invention,the base station 14 continuously transmits a pilot code 40 to all of thesubscriber units 16 located within the transmitting range of the basestation 14. The pilot code 40 is a spreading code which carries no databits. The pilot code 40 is used for subscriber unit 16 acquisition andsynchronization, as well as for determining the parameters of theadaptive matched filter used in the receiver.

[0034] The subscriber unit 16 must acquire the pilot code 40 transmittedby the base station 14 before it can receive or transmit any data.Acquisition is the process whereby the subscriber unit 16 aligns itslocally generated spreading code with the received pilot code 40. Thesubscriber unit 16 searches through all of the possible phases of thereceived pilot code 40 until it detects the correct phase, (thebeginning of the pilot code 40).

[0035] The subscriber unit 16 then synchronizes its transmit spreadingcode to the received pilot code 40 by aligning the beginning of itstransmit spreading code to the beginning of the pilot code 40. Oneimplication of this receive and transmit synchronization is that thesubscriber unit 16 introduces no additional delay as far as the phase ofthe spreading codes are concerned. Accordingly, as shown in FIG. 3, therelative delay between the pilot code 40 transmitted from the basestation 14 and the subscriber unit's transmit spreading code 42 receivedat the base station 14 is 2Δt, which is solely due to the round trippropagation delay.

[0036] In the preferred embodiment, the pilot code is 29,877,120 chipsin length and takes approximately 2 to 5 seconds to transmit, dependingon the spreading factor. The length of the pilot code 40 was chosen tobe a multiple of the data symbol no matter what kind of data rate orbandwidth is used. As is well known by those of skill in the art, alonger pilot code 40 has better randomness properties and the frequencyresponse of the pilot code 40 is more uniform. Additionally, a longerpilot code 40 provides low channel cross correlation, thus increasingthe capacity of the system 10 to support more subscriber units 16 withless interference. The use of a long pilot code 40 also supports agreater number of random short codes. For synchronization purposes, thepilot code 40 is chosen to have the same period as all of the otherspreading codes used by the system 10. Thus, once a subscriber unit 16acquires the pilot code 40, it is synchronized to all other signalstransmitted from the base station 14.

[0037] During idle periods, when a call is not in progress or pending,the subscriber unit 16 remains synchronized to the base station 14 byperiodically reacquiring the pilot code 40. This is necessary for thesubscriber unit 16 to receive and demodulate any downlink transmissions,in particular paging messages which indicate incoming calls.

[0038] When a communication link is desired, the base station 14 mustacquire the signal transmitted from the subscriber unit 16 before it candemodulate the data. The subscriber unit 16 must transmit an uplinksignal for acquisition by the base station 14 to begin establishing thetwo-way communication link. A critical parameter in this procedure isthe transmission power level of the subscriber unit 16. A transmissionpower level that is too high can impair communications in the wholeservice area, whereas a transmission power level that is too low canprevent the base station 14 from detecting the uplink signal.

[0039] In a first embodiment of the present invention the subscriberunit 16 starts transmitting at a power level guaranteed to be lower thanwhat is required and increases transmission power output until thecorrect power level is achieved. This avoids sudden introduction of astrong interference, hence improving system 10 capacity.

[0040] The establishment of a communication channel in accordance withthe present invention and the tasks performed by the base station 14 anda subscriber unit 16 are shown in FIG. 4. Although many subscriber units16 may be located within the operating range of the base station 14,reference will be made hereinafter to a single subscriber unit 16 forsimplicity in explaining the operation of the present invention.

[0041] The base station 14 begins by continuously transmitting aperiodic pilot code 40 to all subscriber units 16 located within theoperating range of the base station 14 (step 100). As the base station14 transmits the pilot code 40 (step 100), the base station 14 searches(step 101) for an “access code” 42 transmitted by a subscriber unit 16.The access code 42 is a known spreading code transmitted from asubscriber unit 16 to the base station 14 during initiation ofcommunications and power ramp-up. The base station 14 must searchthrough all possible phases (time shifts) of the access code 42transmitted from the subscriber unit 16 in order to find the correctphase. This is called the “acquisition” or the “detection” process (step101). The longer the access code 42, the longer it takes for the basestation 14 to search through the phases and acquire the correct phase.

[0042] As previously explained, the relative delay between signalstransmitted from the base station 14 and return signals received at thebase station 14 corresponds to the round trip propagation delay 2Δt. Themaximum delay occurs at the maximum operating range of the base station14, known as the cell boundary. Accordingly, the base station 14 mustsearch up to as many code phases as there are in the maximum round trippropagation delay, which is typically less code phases than there are ina code period.

[0043] For a data rate Rb and spreading code rate Rc, the ratio L=Rc/Rbis called the spreading factor or the processing gain. In the preferredembodiment of the present invention, the cell boundary radius is 30 km,which corresponds to approximately between 1000 and 2500 code phases inthe maximum round trip delay, depending on the processing gain.

[0044] If the base station 14 has not detected the access code aftersearching through the code phases corresponding to the maximum roundtrip delay the search is repeated starting from the phase of the pilotcode 40 which corresponds to zero delay (step 102).

[0045] During idle periods, the pilot code 40 from the base station 14is received at the subscriber unit 16 which periodically synchronizesits transmit spreading code generator thereto (step 103). Ifsynchronization with the pilot code 40 is lost, the subscriber unit 16reacquires the pilot code 40 and resynchronizes (step 104).

[0046] When it is desired to initiate a communication link, thesubscriber unit 16 starts transmitting the access code 42 back to thebase station 14 (step 106). The subscriber unit 16 continuouslyincreases the transmission power while retransmitting the access code 42(step 108) until it receives an acknowledgment from the base station 14.The base station 14 detects the access code 42 at the correct phase oncethe minimum power level for reception has been achieved (step 110). Thebase station 14 subsequently transmits an access code detectionacknowledgment signal (step 112) to the subscriber unit 16. Uponreceiving the acknowledgment, the subscriber unit ceases thetransmission power increase (step 114). With the power ramp-upcompleted, closed loop power control and call setup signaling isperformed (step 116) to establish the two-way communication link.

[0047] Although this embodiment limits subscriber unit 16 transmissionpower, acquisition of the subscriber unit 16 by the base station 14 inthis manner may lead to unnecessary power overshoot from the subscriberunit 16, thereby reducing the performance of the system 10.

[0048] The transmission power output profile of the subscriber unit 16is shown in FIG. 5. At t₀, the subscriber unit 16 starts transmitting atthe starting transmission power level P₀, which is a power levelguaranteed to be less than the power level required for detection by thebase station 14. The subscriber unit 16 continually increases thetransmission power level until it receives the detection indication fromthe base station 14. For the base station 14 to properly detect theaccess code 42 from the subscriber unit 16 the access code 42 must: 1)be received at a sufficient power level; and 2) be detected at theproper phase. Accordingly, referring to FIG. 5, although the access code42 is at a sufficient power level for detection by the base station 14at t_(p), the base station 14 must continue searching for the correctphase of the access code 42 which occurs at t_(A).

[0049] Since the subscriber unit 16 continues to increase the outputtransmission power level until it receives the detection indication fromthe base station 14, the transmission power of the access code 42exceeds the power level required for detection by the base station 14.This causes unnecessary interference to all other subscriber units 16.If the power overshoot is too large, the interference to othersubscriber units 16 may be so severe as to terminate ongoingcommunications of other subscriber units 16.

[0050] The rate that the subscriber unit 16 increases transmission powerto avoid overshoot may be reduced, however, this results in a longercall setup time. Those of skill in the art would appreciate thatadaptive ramp-up rates can also be used, yet these rates haveshortcomings and will not appreciably eliminate power overshoot in allsituations.

[0051] The preferred embodiment of the present invention utilizes “shortcodes” and a two-stage communication link establishment procedure toachieve fast power ramp-up without large power overshoots. The spreadingcode transmitted by the subscriber unit 16 is much shorter than the restof the spreading codes (hence the term short code), so that the numberof phases is limited and the base station 14 can quickly search throughthe code. The short code used for this purpose carries no data.

[0052] The tasks performed by the base station 14 and the subscriberunit 16 to establish a communication channel using short codes inaccordance with the preferred embodiment of the present invention areshown in FIGS. 6A and 6B. During idle periods, the base station 14periodically and continuously transmits the pilot code to all subscriberunits 16 located within the operating range of the base station 14 (step150). The base station 14 also continuously searches for a short codetransmitted by the subscriber unit 16 (step 152). The subscriber unit 16acquires the pilot code and synchronizes its transmit spreading codegenerator to the pilot code. The subscriber unit 16 also periodicallychecks to ensure it is synchronized. If synchronization is lost, thesubscriber unit 16 reacquires the pilot signal transmitted by the basestation (step 156).

[0053] When a communication link is desired, the subscriber unit 16starts transmitting a short code at the minimum power level P₀ (step158) and continuously increases the transmission power level whileretransmitting the short code (step 160) until it receives anacknowledgment from the base station 14 that the short code has beendetected by the base station 14.

[0054] The access code in the preferred embodiment, as previouslydescribed herein, is approximately 30 million chips in length. However,the short code is much smaller. The short code can be chosen to be anylength that is sufficiently short to permit quick detection. There is anadvantage in choosing a short code length such that it divides theaccess code period evenly. For the access code code described herein,the short code is preferably chosen to be 32, 64 or 128 chips in length.Alternatively, the short code may be as short as one symbol length, aswill be described in detail hereinafter.

[0055] Since the start of the short code and the start of the accesscode are synchronized, once the base station 14 acquires the short code,the base station 14 knows that the corresponding phase of the accesscode is an integer multiple of N chips from the phase of the short codewhere N is the length of the short code. Accordingly, the base station14 does not have to search all possible phases corresponding to themaximum round trip propagation delay.

[0056] Using the short code, the correct phase for detection by the basestation 14 occurs much more frequently. When the minimum power level forreception has been achieved, the short code is quickly detected (step162) and the transmission power overshoot is limited. The transmissionpower ramp-up rate may be significantly increased without concern for alarge power overshoot. In the preferred embodiment of the presentinvention, the power ramp-up rate using the short code is 1 dB permillisecond.

[0057] The base station 14 subsequently transmits a short code detectionindication signal (step 164) to the subscriber unit 16 which enters thesecond stage of the power ramp-up upon receiving this indication. Inthis stage, the subscriber unit 16 ceases transmitting the short code(step 166) and starts continuously transmitting a periodic access code(step 166). The subscriber unit 16 continues to ramp-up its transmissionpower while transmitting the access code, however the ramp-up rate isnow much lower than the previous ramp-up rate used with the short code(step 168). The ramp-up rate with the access code is preferably 0.05 dBper millisecond. The slow ramp-up avoids losing synchronization with thebase station 14 due to small changes in channel propagationcharacteristics.

[0058] At this point, the base station 14 has detected the short code atthe proper phase and power level (step 162). The base station 14 mustnow synchronize to the access code which is the same length as all otherspreading codes and much longer than the short code. Utilizing the shortcode, the base station 14 is able to detect the proper phase of theaccess code much more quickly. The base station 14 begins searching forthe proper phase of the access code (step 170). However, since the startof the access code is synchronized with the start of the short code, thebase station 14 is only required to search every N chips; where N thelength of the short code. In summary, the base station 14 quicklyacquires the access code of the proper phase and power level by: 1)detecting the short code; and 2) determining the proper phase of theaccess code by searching every N chips of the access code from thebeginning of the short code.

[0059] If the proper phase of the access code has not been detectedafter searching the number of phases in the maximum round trip delay thebase station 14 restarts the search for the access code by searchingevery chip instead of every N chips (step 172). When the proper phase ofthe access code has been detected (step 174) the base station 14transmits an access code detection acknowledgment (step 176) to thesubscriber unit 16 which ceases the transmission power increase (step178) upon receiving this acknowledgment. With the power ramp-upcompleted, closed loop power control and call setup signaling isperformed (step 180) to establish the two-way communication link.

[0060] Referring to FIG. 7, although the starting power level P₀ is thesame as in the prior embodiment, the subscriber unit 16 may ramp-up thetransmission power level at a much higher rate by using a short code.The short code is quickly detected after the transmission power levelsurpasses the minimum detection level, thus minimizing the amount oftransmission power overshoot.

[0061] Although the same short code may be reused by the subscriber unit16, in the preferred embodiment of the present invention the short codesare dynamically selected and updated in accordance with the followingprocedure. Referring to FIG. 8, the period of the short code is equal toone symbol length and the start of each period is aligned with a symbolboundary. The short codes are generated from a regular length spreadingcode. A symbol length portion from the beginning of the spreading codeis stored and used as the short code for the next 3 milliseconds. Every3 milliseconds, a new symbol length portion of the spreading codereplaces the old short code. Since the spreading code period is aninteger multiple of 3 milliseconds, the same short codes are repeatedonce every period of the spreading code.

[0062] Periodic updating of the short code averages the interferencecreated by the short code over the entire spectrum. A detaileddescription of the selection and updating of the short codes is outsidethe scope of this invention. However, such a detailed description isdisclosed in the related application U.S. Patent Appln. entitled CodeDivision Multiple Access (CDMA) System and Method.

[0063] A block diagram of the base station 14 is shown in FIG. 9.Briefly described, the base station 14 comprises a receiver section 50,a transmitter section 52 and a diplexer 54. An RF receiver 56 receivesand down-converts the RF signal received from the diplexer 54. Thereceive spreading code generator 58 outputs a spreading code to both thedata receiver 60 and the code detector 62. In the data receiver 60, thespreading code is correlated with the baseband signal to extract thedata signal which is forwarded for further processing. The receivedbaseband signal is also forwarded to the code detector 62 which detectsthe access code or the short code from the subscriber unit 16 andadjusts the timing of the spreading code generator 58 to establish acommunication channel 18.

[0064] In the transmitter section 52 of the base station 14, thetransmit spreading code generator 64 outputs a spreading code to thedata transmitter 66 and the pilot code transmitter 68. The pilot codetransmitter 68 continuously transmits the periodic pilot code. The datatransmitter 66 transmits the short code detect indication and accesscode detect acknowledgment after the code detector 62 has detected theshort code or the access code respectively. The data transmitter alsosends other message and data signals. The signals from the datatransmitter 66 and the pilot code transmitter 68 are combined andup-converted by the RF transmitter 70 for transmission to the subscriberunits 16.

[0065] A block diagram of the subscriber unit 16 is shown in FIG. 10.Briefly described, the subscriber unit 16 comprises a receiver section72, a transmitter section 74 and a diplexer 84. An RF receiver 76receives and down-converts the RF signal received from the diplexer 84.A pilot code detector 80 correlates the spreading code with the basebandsignal to acquire the pilot code transmitted by the base station 16. Inthis manner, the pilot code detector 80 maintains synchronization withthe pilot code. The receiver spreading code generator 82 generates andoutputs a spreading code to the data receiver 78 and the pilot codedetector 80. The data receiver 78 correlates the spreading code with thebaseband signal to process the short code detect indication and theaccess code detect acknowledgment transmitted by the base station 16.

[0066] The transmitter section 74 comprises a spreading code generator86 which generates and outputs spreading codes to a data transmitter 88and a short code and access code transmitter 90. The short code andaccess code transmitter 90 transmits these codes at different stages ofthe power ramp-up procedure as hereinbefore described. The signalsoutput by the data transmitter 88 and the short code and access codetransmitter 90 are combined and up-converted by the RF transmitter 92for transmission to the base station 14. The timing of the receiverspreading code generator 82 is adjusted by the pilot code detector 80through the acquisition process. The receiver and transmitter spreadingcode generators 82, 86 are also synchronized.

[0067] An overview of the ramp-up procedure in accordance with thepreferred current invention is summarized in FIGS. 11A and 11B. The basestation 14 transmits a pilot code while searching for the short code(step 200). The subscriber unit 16 acquires the pilot code transmittedfrom the base station 14 (step 202), starts transmitting a short codestarting at a minimum power level P₀ which is guaranteed to be less thanthe required power, and quickly increases transmission power (step 204).Once the received power level at the base station 14 reaches the minimumlevel needed for detection of the short code (step 206) the base station14 acquires the correct phase of the short code, transmits an indicationof this detection, and begins searching for the access code (step 208).Upon receiving the detection indication, the subscriber unit 16 ceasestransmitting the short code and starts transmitting an access code. Thesubscriber unit 16 initiates a slow ramp-up of transmit power whilesending the access code (step 210). The base station 14 searches for thecorrect phase of the access code by searching only one phase out of eachshort code length portion of the access code (step 212). If the basestation 14 searches the phases of the access code up to the maximumround trip delay and has not detected the correct phase, the search isrepeated by searching every phase (step 214). Upon detection of thecorrect phase of the access code by the base station 14, the basestation 14 sends an acknowledgment to the subscriber unit 16 (step 216).Reception of the acknowledgment by the subscriber unit 16 concludes theramp-up process. A closed loop power control is established, and thesubscriber unit 16 continues the call setup process by sending relatedcall setup messages (step 218).

[0068] Although the invention has been described in part by makingdetailed reference to the preferred embodiment, such detail is intendedto be instructive rather than restrictive. It will be appreciated bythose skilled in the art that many variations may be made in thestructure and mode of operation without departing from the spirit andscope of the invention as disclosed in the teachings herein.

What is claimed is:
 1. A base station for controlling transmission powerduring the establishment of communications between a base station andanother communicating unit, comprising: means for detecting a periodicsignal having an initial predetermined power level, which is repeatedlysent to the base station at increasing power levels; means fortransmitting a power control signal when a sufficient periodic signalhaving a sufficient power for detection is received; and means forreceiving an access signal having a power level controlled by said powercontrol signal.
 2. The base station of claim 1 wherein said initialpredetermined power level is lower than a power level required fordetection by said base station.
 3. The base station of claim 1 whereinsaid periodic signal comprises a short code and said access signalcomprises an access code, which is an integer multiple of the shortcode.
 4. The base station of claim 3 wherein the correct phase of theaccess code is obtained by searching only one phase out of a short codelength portion of the access code.
 5. The base station of claim 4further comprising means for transmitting an acknowledgment when thecorrect phase of the access code is detected.
 6. The base station ofclaim 4 further comprising means for performing a subsequent search bysearching every phase when a correct phase has not been detected after agiven interval.
 7. The base station of claim 6 wherein said interval ismeasured starting at a beginning of the initial search.
 8. The basestation of claim 3 wherein the control signal is generated by saidcontrol signal generating means when the correct phase of the short codeis detected.